In general, when a driven machine which is driven by a driving machine is driven directly or indirectly through a belt means, the rotative speed of the driven machine increases according to the increment of increase in the rotative speed of the driving machine.
However, in many cases, a constant rotative speed is required, e.g. where the driven machine is a power steering pump or a compressor for use in a cooler which is driven by an internal combustion engine acting as a driving machine regardless of the rotational speed and the variance of the load of the internal combustion engine.
In order to satisfy the above described requirement, a fluid power transmission coupling has been used.
The usual fluid coupling device is provided with a rotor fixed to a driving shaft, and a casing spaced from the rotor at a short distance, which acts as a driven shaft. A fluid having high viscosity such as silicon oil is provided as a working fluid sealed in a chamber defined between the rotor and the casing.
Conventional fluid coupling devices having the above described structure are defective as follows. Firstly, a large amount of torque cannot be transmitted thereby since the shaft torque is transmitted by using the viscosity resistance of the sealed fluid, and a larger outer diameter is required in such a power transmission device for transmitting a large torque thereby.
Second, since the rotative speed of the driven shaft is determined due to the load of the driven side, this device cannot be easily designed. Namely, a large load variation is not desirable on the driven side thereof.
Thirdly, since slippage occurs between the rotor and the casing even at a low rotative speed, transmission efficiency is not good.
Fourthly, since the viscosity of the viscous fluid is varied according to ambient temperature, the operation characteristics are non-uniform for that additional reason.